Radiopharmaceuticals may be used as diagnostic or therapeutic agents by virtue of the physical properties of their constituent radionuclides. Thus, their utility is not based on any pharmacologic action per se. Most clinically-used drugs of this class are diagnostic agents incorporating a gamma-emitting nuclide which, because of physical, metabolic or biochemical properties of its coordinated ligands, localizes in a specific organ after intravenous injection. The resultant images can reflect organ structure or function. These images are obtained by means of a gamma camera that detects the distribution of ionizing radiation emitted by the radioactive molecules.
In radioimaging, the radiolabel is a gamma-radiation emitting radionuclide and the radiotracer is located using a gamma-radiation detecting camera (this process is often referred to as gamma scintigraphy). The imaged site is detectable because the radiotracer is chosen either to localize at a pathological site (termed positive contrast); alternatively, the radiotracer is chosen specifically not to localize at such pathological sites (termed negative contrast).
Many of the procedures presently conducted in the field of nuclear medicine involve radiopharmaceuticals which provide diagnostic images of blood flow (perfusion) in the major organs and in tumors. The regional uptake of these radiopharmaceuticals within the organ of interest is proportional to flow; high flow regions will display the highest concentration of radiopharmaceutical, while regions of little or no flow have relatively low concentrations. Diagnostic images showing these regional differences are useful in identifying areas of poor perfusion, but do not provide metabolic information of the state of the tissue within the region of apparently low perfusion.
It is well known that tumors often have regions within their mass which are hypoxic. These result when the rapid growth of the tumor is not matched by the extension of tumor vasculature. A radiopharmaceutical which localizes preferentially within regions of hypoxia could be used to provide images which are useful in the diagnosis and management of therapy of tumors, as suggested by Champman, “Measurement of Tumor Hypoxia by Invasive and Non-Invasive Procedures—A Review of Recent Clinical Studies”, Radiother. Oncol., 20(S1), 13-19 (1991). Additionally, a compound which localizes within the hypoxic region of tumors, but is labeled with a radionuclide with suitable alpha- or beta-emissions could be used for the internal radiotherapy of tumors. In the brain or heart, hypoxia typically follows ischemic episodes produced by, for example, arterial occlusions or by a combination of increased demand and insufficient flow.
However, many radionuclides are less than ideal for routine clinical use. For example, the positron-emitting isotopes (such as 18F) are cyclotron-produced and short-lived, thus requiring that isotope production, radiochemical synthesis, and diagnostic imaging be performed at a single site or region. The costs of procedures based on positron-emitting isotopes are very high, and there are very few of these centers worldwide. While 123I-radiopharmaceuticals may be used with widely-available gamma camera imaging systems, 123I has a 13-hour half-life (which restricts the distribution of radiopharmaceuticals based on this isotope) and is expensive to produce. Nitroimidazoles labeled with 3H are not suitable for in vivo clinical imaging and can be used for basic research studies only.
A number of factors must be considered for optimal radioimaging in humans. To maximize the efficiency of detection, a radionuclide that emits gamma energy in the 100 to 200 keV range is preferred. To minimize the absorbed radiation dose to the patient, the physical half-life of the radionuclide should be as short as the imaging procedure will allow. To allow for examinations to be performed on any day and at any time of the day, it is advantageous to have a source of the radionuclide always available at the clinical site.
A variety of radionuclides are known to be useful for radioimaging, including Ga-67, Tc-99m, In-111, I-123, and I-131. The preferred radioisotope for medical imaging is Tc-99m. Its 140 keV gamma-photon is ideal for use with widely-available gamma cameras. It has a short (6 hour) half life, which is desirable when considering patient dosimetry. Tc-99m is readily available at relatively low cost through commercially-produced 99Mo/Tc-99m generator systems. As a result, over 80% of all radionuclide imaging studies conducted worldwide utilize Tc-99m. See generally Reedijk J. “Medicinal Applications of heavy-metal compounds” Curr. Opin. Chem. Biol. (1999) 3(2): 236-240; and Hom, R. K., Katzenellenbogen, J. A. “Technetium-99m-labeled receptor-specific small-molecule radiopharmaceuticals: recent developments and encouraging results” Nuc. Med. and Biol. (1997) 24: 485-498. These advantages, coupled with the fact that Single Photon Emission Computed Tomography cameras are optimized for the 140 keV energy of Tc-99m, clearly demonstrate the superiority of Tc-99m-labeled imaging agents.
Recently, a new Tc(I) labeling system has been developed. Aberto, R., Schibli, R., Egli, A., Schubiger, A. P., Abram, U., Kaden, T. A. “A Novel Organometallic Aqua Complex of Technetium for the Labeling of Biomolecules: Synthesis of [99mTc(OH2)3(CO)3]+ from [99mTcO4]− in Aqueous Solution and Its Reaction with a Bifunctional Ligand” J. Am. Chem. Soc. (1998) 120: 7987-7988; and Alberto, R., Schibli, R., Daniela, A., Schubiger, A. P., Abram, U., Abram, S., Kaden, T. A. “Application of technetium and rhenium carbonyl chemistry to nuclear medicine—Preparation of [Net4]2[TcCl3(CO)3] from [NBu4][TcO4] and structure of [NEt4][Tc2(u-Cl)3(CO)6]; structures of the model complexes [NEt4][Re2(u-OEt)2(u-OAc)(CO)6] and [ReBr({-CH2S(CH2)2Cl}2(CO)3]” Transition Met. Chem. (1997) 22: 597-601. This system takes advantage of the organometallic Tc(I) carbonyl chemistry. Importantly, the chemistry of [99mTc(OH2)3(CO)3]+ has been elucidated and simplified to the point where the methods are routine and offer a practical alternative to the currently employed Tc(V) chemistry. In contrast to the highly reactive Tc(V)-oxo cores, where the chemistry is sometimes unpredictable and includes labeling cleanup steps, the Tc(I) method offers an attractive labeling alternative. However, unlike the Tc(V)-oxo core, the Tc(I)(CO)3+ core limits the number of possible coordination geometries available for Tc due to the presence of the three carbonyl groups. The facial arrangement of carbonyl ligands around the metal center also impose steric constraints on the binding possibilities of the remaining three sites.
Moreover, the [99mTc(OH2)3(CO)3]+ complex can be readily prepared in saline under 1 atom of carbon monoxide (CO). This water and air stable Tc(I) complex is a practical precursor to highly inert Tc(I) complexes, due in part to the d6 electron configuration of the metal center. As already pointed out, the preparation of the organometallic tris(aquo) ion is simple and straightforward, allowing for convenient manipulation and product formation. Substitution of the labile H2O ligands has been shown to leave the Tc(CO)3+ core intact. This stable core has the additional advantage of being smaller and less polar than the routinely employed Tc(V)-oxo systems. This characteristic could be advantageous in biologically relevant systems where the addition of the metal center effects the size, shape, and potentially the bioactivity of the compounds.
Although various chelators are currently employed in the binding of technetium, all of these tracers suffer from one or more disadvantages which render them less than ideal: HYNIC requires coligands; MAG3 may be only used with the Tc(V)-oxo species; EDTA/DTPA is used primarily with Tc(V)-oxo and its ability to retain label is poor. Hence, additional Technetium-99m chelators are needed. Novel radiolabeled chelators that display rapid, efficient labeling and demonstrate superior labeling retention for both Tc(V)-oxo and Tc(I)-tricarbonyl cores without the use of coligands are attractive candidates for clinical evaluation as potential chelators for biologically relevant molecules.